1. Technical Field
The present invention relates to a square-sectioned synthetic resin container, particularly to such a container capable of effectively compensating for lowering of the strength due to the reduction of the container wall thickness.
2. Background Art
Recently, containers made of synthetic resin (“synthetic resin containers”) as represented by PET bottles are widely used as storage for food, beverages, cosmetics, medicines or the like. This is because synthetic resin containers are light in weight and easy to handle, and preserve transparency to exhibit refined appearance comparable with glass bottles, besides that they can be produced at relatively low cost.
Synthetic resin container has relatively low mechanical strength against external force so that, for example, if one holds the body of the container by hand for pouring out its contents, the body part held by hand readily undergoes deformation. Thus, for this type of synthetic resin container, in order to improve the mechanical resistance (buckling strength, rigidity, etc.) against external forces, it is customary to suitably adjust the container wall thickness and provide lengthwise or crosswise ribs around the body, or to inscribe a narrow groove (waist) having an inwardly projecting trapezoidal profile around the body.
From the viewpoint of effective use of natural resources and reduction of wastes, there is recently an increasing demand for reducing the amount of resin used for one container, by reducing the wall thickness (or the weight) of the container. In order to satisfy such demand, it would be inevitable for the synthetic resin containers to have a reduced strength. Specifically, in the case of a waist-inscribed synthetic resin container having a square cross-section, there is often adopted an arrangement wherein the depth of the waist groove at the pillar portions is made smaller than the depth of the grooves at the wall portions, in consideration of buckling. Despite such an arrangement, however, if a load is applied onto the container from the top portion downwards, stresses tend to concentrate at the groove portions inscribed on the pillar portions (i.e., at the corner portions of the container). Such stress concentration is marked particularly at the joint of the surfaces where the waist is inflected to project convexly inwards of the container, thereby causing buckling of the container starting from the joint in question, and an effective solution of such problem is therefore highly demanded.